FIG. 1 shows a ducted fan gas turbine engine 10 comprising in axial flow series: an air intake 12, a propulsive fan 14 having a plurality of fan blades 16, an intermediate pressure compressor 18, a high-pressure compressor 20, a combustor 22, a high-pressure turbine 24, an intermediate pressure turbine 26, a low-pressure turbine 28 and a core exhaust nozzle 30. A nacelle 32 generally surrounds the engine 10 and defines the intake 12, a bypass duct 34 and a bypass exhaust nozzle 36. The engine has a principal axis of rotation 31.
Air entering the intake 12 is accelerated by the fan 14 to produce a bypass flow and a core flow. The bypass flow travels down the bypass duct 34 and exits the bypass exhaust nozzle 36 to provide the majority of the propulsive thrust produced by the engine 10. The core flow enters in axial flow series the intermediate pressure compressor 18, high pressure compressor 20 and the combustor 22, where fuel is added to the compressed air and the mixture burnt. The hot combustion products expand through and drive the high, intermediate and low-pressure turbines 24, 26, 28 before being exhausted through the nozzle 30 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 24, 26, 28 respectively drive the high and intermediate pressure compressors 20, 18 and the fan 14 by concentric interconnecting shafts 38, 40, 42.
The turbines and compressors are constructed from axial arranged pairs of nozzle guide vanes and blades which are relatively rotatable. The vanes and blades each comprise aerofoil portions and platforms located at opposing ends of the aerofoils which define the main gas path. Thus, a nozzle guide vane may have radially inner and outer platforms with the aerofoil extending therebetween, and the blades may have inner platforms which are rotatably separated from the nozzle guide vane platforms. The tips of the blades may be so-called shrouded blades having integral shrouds which circumferentially combine to provide an annular wall which rotates with the blades. Such are arrangements and others are well known in the art.
A result of having relative rotation between the blades and vanes means that the main gas path wall is necessarily axially segmented. Axial segmentation of the gas path wall also aids construction and assembly of the engines amongst other advantages.
FIG. 2 shows a partial streamwise section of an intermediate pressure turbine 210 as highlighted in FIG. 1. The turbine includes a blade 212 and downstream vane 214 which forms part of the following vane-blade stage. The turbine blade 212 and vane 214 have aerofoil portions located within the main gas path 216. The blade 212 is a shrouded blade meaning that the tip of the blade terminates with a shroud platform 218 which circumferentially engages with adjacent shroud platforms to provide a full annulus. A seal segment 220 is positioned radially outboard of the shroud platform 218.
The downstream vane 214 includes an outer platform 222 which is integrally formed with the aerofoil portion. Both the seal segment 220 and outer vane platform 222 are held in a substantially stationary relation to the engine casing 224. The seal segment 220 and outer platform 222 are separate components which are axially separate but attached to one another to provide a substantially continuous gas path wall.
The seal segment 220 and outer platform 222 are attached to one another and to the engine casing 224. The attachments used can be any suitable type but are typically male and female connectors in the form of hook and grooves or so-called birdsmouth couplings 226. The birdsmouth couplings 226 include corresponding hook and groove formations on each of the components which engage axially with each other to provide an attachment therebetween. The birdsmouth couplings 226 principally provide radial restraint to the components with axial restraint provided by other means. It will be appreciated that hooks and grooves extend circumferentially around the seal shroud platform to the extent required to provide the radial restraint.
A problem with providing birdsmouth coupling 226 is that they have a tendency to leak during the inevitable relative movement between the axially adjacent parts. To combat this, the birdsmouth attachments 226 are often accompanied by one or more seals. Such a seal is shown in FIG. 2 and is in the form of a baffle seal 228.
The present invention seeks to provide an improved seal arrangement.